Method of hot compacting titanium powder

ABSTRACT

Hot isostatic compacting of titanium or titanium-base alloy powder to a product having controlled interstitials and microstructure and possessing outstanding end properties, low porosity and high density and one which does not require subsequent forging.

t mte States atent 1 1 3,729,971 Gurganus et al. 1 1 May 1, 1973 METHODOF HOT COMPACTING [56] References Cited TITANIUM POWDER UNITED STATESPATENTS [75] Inven r Th ma rg n A n; o 2,932,882 4/1960 Kelly ..29/420don K. Turnbull, Rocky River; Allen 3,052,976 9/1962 Rennhack ..29/420.5X Montgomery, Cleveland, a f 3,475,142 10/1969 Abkowitz et a] ..29/420.5X Ohio 3,279,917 10/1966 Ballard et a]. ..75/226 3,284,195 11/1966Googin et al. ..75/226 [73] Assignee: Aluminum Company of America,3,390,985 7/1 68 Cr eni t 5/226 X Pittsburgh, p 3,681,037 8/1972Abkowitz et al ..75/226 x [22] Filed: 1971 Primary Examiner-Charles W.Lanham [21] Appl. No.: l27,457 Assistant Examiner-D. C. Reiley, III

Attorney-Abram W. Hatcher [52] US. Cl ..72/226, 29/4205, 29/DIG. 31,[57] ABSTRACT 29/DIG. 45 51 Int. Cl. ..B22f 3/24 505mm compactmg of [58]Field of Search ..29/420.5, 420, DIG. 31, alloy powder a product havmgmmmned 29/DIG. 45; 75/226 tials and microstructure and possessingoutstanding end properties, low porosity and high density and one whichdoes not require subsequent forging.

15 Claims, 3 Drawing Figures Patented May 1 1973 FIG. 2.

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moms a. GURGA/VUS, ammo/v K. TURNBULL and ALL EN M. MON TGOMER r cwumm'Attorney METHOD OF HOT COMPACTING TITANIUM POWDER BACKGROUND OF THEINVENTION This invention relates to hot isostatic compacting of titaniumpowder. More particularly, it relates to the simultaneous use of hightemperatures and pressures to compress titanium powder to ahigh-density, controlled-interstitial product which does not requiresubsequent forging to improve its microstructure or properties. The termtitanium as used herein includes titanium or titanium-base alloyscontaining at least 50 percent by weight titanium.

Despite continued efforts on the part of research and developmentpersonnel, titanium powder metallurgy parts have not reached broadcommercial acceptance. The primary unsolved problem until now seems tohave been finding a way to produce titanium powder metallurgy parts withproperties substantially equivalent to those of titanium forged parts,specifically, commercially acceptable properties, that is, thoseacceptable for complex parts under most commercial specifications. Ithas been difficult to obtain parts of the high density, low porosity andhigh elongation and reduction in area required by wrought titaniumspecifications. One way in which this has been attempted, but withoutcomplete success, has been simultaneous compacting and sintering oftitanium powder compacts. For example, a cold compact of loosely pressedtitanium powder contained within a suitable mold such as formed by ahigh temperature-withstanding material has been isostatically compactedat high temperature to produce a product of a simple shape, such as acylinder or rectangular block. HOwever, such a product was not producedusing the necessary controls on interstitials and microstructure.Therefore, it required subsequent forging to improve property levels.Even by such a hot isostatic compact-forge approach it has still beendifficult to obtain typical wrought titanium properties.

SUMMARY OF THE INVENTION After extended investigation we have found thatcontrolling interstitial content, that is, levels of oxygen, nitrogen,hydrogen and carbon, especially oxygen, in the titanium; maintaining forthe titanium an alphabeta microstructure substantially fieg ofalpha-outlined prior beta grains and Widmanstatten patterned alphaparticles; keeping the temperature below the beta transus temperature ofthe alloy undergoing the hot isostatic compacting; and using anintermediate pressure transfer medium between the compacting gas and thepowder being compacted and shaped permit attainment of the desired highdensity, low porosity, high yield and tensile strength, high elongationand reduction of the area, as well as an acceptable controlled limit forfinal interstitial content, such that the resulting shaped products arecommercially acceptable.

According to our invention, to produce a shaped article or product ofthe desired controlled interstitial content it is desirable to maintainthe titanium at a controlled interstitial content throughout theprocess. The powder which we compact may be prepared by conventionalmethods, for example, by the process of U. S. Pat. No. 2,628,786, or bymachining scrap forgings into chips, using cold water as lubricant andas coolant to inhibit interstitial formation, and then cleaning, dryingand reducing the resultant chips on alcohol by attrition to powder, forexample, according to U. S. Pat. No. 3,496,036, using a heliumatmosphere. The powder may then be screened to the desired size, forexample -40, +200 mesh. Regardless of the method used, however, theheating and working cycle must be such that, as mentioned above, thepowder continues to have a microstructure substantially free ofalpha-outlined prior beta grains and Widmanstitten patterned alphaparticles. We prefer to start with a so-called cold compact, preferablyof a density of at least about 60 percent of theoretical, which may beprepared by compacting by conventional methods, one of these beingpouring titanium powder stock of about 15-30 percent of theoreticaldensity into a rubber or plastic-type mold having a shape similar tothat desired for the final part, drawing a vacuum on the powdered-filledmold, sealing the mold, placing the mold in a cold isostatic compactionunit and compacting same to the starting density. When this procedure isused for forming the compacted powder compressed according to thepresent invention, the mold may be stripped from the compact afterremoval of the compact from the cold isostatic compaction unit. Ifloose, non-compacted powder is used at the start of the hot isostaticcompacting, the powder is first poured into a container which mayresemble the desired end-product shape, allowing for shrinkage. In thisinstance, the container may serve as the intermediate pressure transfermedium, provided it is a material adapted for compaction at the requiredhigh temperature and pressure used according to the invention.

According to the present invention, the powder or powder compact isplaced in a hot isostatic compacting unit. It may be in a container. Anintermediate pressure transfer medium is placed around the powder in thedesired surface contour, either before or after it is placed inside thehot isostatic compacting unit. The intermediate pressure transfer mediumused to form a seal or protective layer between the compact and thegaseous or liquid fluid under pressure used in the hot isostaticcompacting unit may be either solid or liquid. Such media tend topromote a uniform pressure distribution against the whole surface of thecompact while it is being hot-compacted under pressure. Examples ofmedia which may be used in the compacting operation according to ourinvention include glass, steel, sand and the like. When in particulateform, they may be either rounded, angular or fibrous. The medium may inthe form of a sheath which is sealed over the surface of the compact,for example, steel, preferably stainless. Molten glass is representativeof suitable liquid media. Particles of sand may be used as theintermediate pressure transfer medium according to our invention.

According to the preferred practice, a vacuum is drawn on the systemcontaining the cold compact surrounded by the intermediate pressuretransfer medium after heating to about 800l000 F, and the system is thensealed prior to isostatically compacting to a high density,advantageously greater than about 97 percent.

One of the advantages of this hot isostatic compacting procedure is thatthe compact may be of almost any desired configuration, including suchhighly complex forms as gears, marine parts, disc ornamental items orthe like. The cold compact is compressed substantially uniformly in alldirections inwardly such that it holds the desired shape. We prefer touse pressures of about 1,000-25,000 psi and temperatures of about l6501850 F. The temperature must be below the beta transus temperature forthe particular alloy being compacted such as will not permit formationof the alphaoutlined prior beta grains and Widmanstatten patterned alphaparticles which our invention does not allow in the titanium and whichprevent attainment according to the invention of the improved ductilitycomparable to that of forged ingot products. The temperature used ispreferably not more than about 200 F below the beta transus temperaturefor the powder being compacted. The product of the controlled hotisostatic compacting procedure employing inter mediate pressure transfermedia according to our invention meets final end-use properties that is,commercially acceptable wrought properties when conventionally heattreated. Subsequent forging is unnecessary.

Titanium which may be compacted according to our process includes, forexample, in addition to substantially pure titanium, titanium-basealloys, for example, of the following compositions, the numbersreferring to percent by weight of the major components other thantitanium (except for minor impurities): 6A1-4V, 6A 1 6V2Sr1, 8A1-1Mo-IV,Al-2.5Sn, 6Al-2 Sn-4Zr6Mo, 6Al2Sn4Zr--2Mo-O.25Si and l3Vl lCr-3A1.

According to our invention the titanium powder is preferably hotisostatically compacted at a sufficient temperature and pressure and fora sufficient time to give the resulting shaped articles or products,after heat treatment, typical forged properties, particularly anelongation and reduction in area heretofore generally unattainable for atitanium powder product, for example, for a 1 inch maximum-thicknessforged specimen after solution heat treatment and annealing, orannealing a minimum elongation of 10 percent and a minimum reduction inarea of 25 percent (after the breaking point when a tensile bar istested according to the standard procedure), or, after solution heattreatment and aging, a minimum elongation of 8 percent and a minimumreduction in area of percent.

As mentioned above, the compacting temperature according to ourinvention should not exceed the beta transus temperature. As known tothose skilled in the art, this is the temperature above which thetitanium or titanium alloy exists chiefly in the beta form. If highertemperatures are used, undesirable alpha-outlined prior beta grains andWidmanstz'itten patterned alpha, overheated structure, such as may causeunsatisfactory end properties, are likely to develop. As alreadyindicated, the temperature employed is preferably within the range offrom 200 F below to the beta transus temperature of the titanium alloybeing compacted. When higher temperatures are used, an undesirablemicrostructure results. Such a microstructures is typical of acrystallographic pattern which indicates low ductility when compared tothe microstructure of a forged ingot product. This undesirablemicrostructure or crystallographic pattern is sometimes characterized byan excessive beta grain size and alpha-outlined prior beta grains, withstraight alpha particles within these grains,

or by what we have referred to hereinabove as alphaoutlined prior betagrains and Widmanstiitten patterned alpha particles. Presence of such apattern generally indicates decreased ductility in the product. Thehigh-temperature form of unalloyed titanium, that is, the beta form, hasa characteristically body-centered cubic structure and exists aboveabout l620 F, the beta transus temperature, whereas the low-temperatureor alpha from has a characteristically hexagonalpacked structure towhich beta transforms below this temperature. The desirable wroughtproperties brought about by our invention are further associated with amicrostructure which comprises equiaxed primary alpha particles within atransformed beta matrix. Use of conventional sintering temperaturesafter cold compacting tends to destroy the desired microstructure andthereby prevent obtaining the high ductility required for commercialproducts.

Use of hot isostatic compacting according to our invention substantiallyprevents any undesirable increase in interstitial content between thestarting powder stage and the final shaped product. The interstitialcontents for a product of the desired properties are therefore generallysubstantially the same for the final shaped article produced by hotisostatic compacting according to the invention as for the startingpowder or preferred cold compact. For optimum results in producing thedesired improved end product properties according to the invention, weprefer that the interstitial content be controlled throughout theprocess such that at least one of the group oxygen, nitrogen, hydrogenand carbon not exceed a particular value, viz., for a Ti-6AI4V alloy foroxygen 2,000 ppm or 0.2 percent, for nitrogen 500 ppm or 0.05 percent,for hydrogen ppm or 0.0125 percent or for carbon 1,000 ppm or 0.1percent, the percent figures referring to percent by weight. That is, weprefer that the content of these interstitials not exceed therequirement of MIL-T--9047.

To decrease porosity problems and to prevent dangerously highinterstitial content in the shaped article produced by hot isostaticcompacting according to the present invention, it may be advisable todegas by heating prior to the hot isostatic compacting step.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of ourinvention, reference will now be made to the drawing, which forms a parthereof.

FIGS. 1 and 2 are schematic representations of titanium powdermetallurgy parts 10 and 12 such as may be produced according to theprocess of this invention.

FIG. 3 is a schematic cut-away cross-sectional drawing of a portion of arepresentative hot isostatic compaction unit which may be used accordingto our invention and which contains therein a titanium powder coldcompact inside a surrounding pressure transfer medium which separates itfrom hot pressurized gas inside the hot isostatic compaction unit. InFIG. 3, a hot isostatic compaction unit is represented by 14, a spacecontaining hot pressurized gas by 16, a powder metallurgy shapeundergoing or to undergo hot isostatic compacting according to theinvention, by 18, an intermediate pressure transfer medium by 20, and acontainer (for example, stainless steel) by 22. According to theinvention, a powder of controlled 0.2 percent oxygen content 18 isplaced in vessel 22 and surrounded by intermediate pressure transfermedium 20. The cold compact pressure medium system is then degassedunder vacuum and sealed. It is then placed in hot isostatic compactionunit 14, the unit sealed and the gas 16 pressurized to the desiredpressure, for example, 1,000 to 25,000 psi, by means of standardpressurizing methods. The gas may be any suitable gas which issubstantially inert to the intermediate pressure transfer medium 20 andthe walls of the hot isostatic compaction unit 14, for example, argon.The heating to the desired hot compacting temperature may be donebefore, during or after gas introduction. The combination of pressureand temperature applied forms the powder to the desired high densityshaped article without the necessity of subsequent sintering.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following example isillustrative of our invention. 7

Chips were machined from a Ti-6A1-4V alloy billet, using cold water aslubricant and coolant to inhibit interstitial pickup, cleaned withalcohol and reduced to powder by attrition under a cooled argonatmosphere. The resultant powders had an oxygen content of 2,000 ppm.They were screened, while still under the argon atmosphere, to 40, +200mesh and then placed in a cylindrical rubber bag mold for coldcompaction. A vacuum was drawn on the bag and the bag sealed. Thepowder-filled bag was placed in a cold isostatic compaction unit andcompacted at 60,000 psi to a density of approximately 68 percent oftheoretical. The cold-compacted sample was removed from the coldisostatic compaction unit and the rubber mold stripped therefrom. It wasthen sheathed in a stainless steel container which served as anintermediate pressure transfer medium, a vacuum was drawn on thecompactcontainer system, and the container sealed. The stainlesssteel-clad cold compact was placed in a hot isostatic compaction unitsuch as that depicted in FIG. 3 for six hours at 1600 F and 1,500 psiand isostatically compacted therein. The resultant hot isostaticallycompacted sample was then quartered, one quarter being retained as hotisostatically compacted, and the remaining three upset either 10, 20 or30 percent on flat dies at 1750 F. Tensile blanks were machined fromeach quarter and solution heat treated and aged as follows: heat onehour at 1750 F, water quench within five seconds, age four hours at 1000F and air cool. Tensile data were taken and compared to minimum forgingcommercial requirements for similar prior art standard forgings. Theseappear in the following table.

TABLE {Tensile properties of solution heat treated and aged sum lltS ofhot lsostatically compacted THEM-4C alloy powdersi l Specification (AMS)4,967 minimums. I Plus.

'ifiiiii' iafi' examination showed the hot is o statically compacted andsolution heat treated and aged samples to be of the desired equiaxialprimary alpha particles within a transformed beta matrix structure. Asindicated in the above table, the properties of the hot isostaticallycompacted samples after solution treatment and aging were well abovecommercial specifications. WTIiE the inventiomhas been described interms bf preferred embodiments, the claims appended hereto are intendedto encompass all embodiments which fall within the spirit of theinvention.

Having thus described our invention and certain embodiments thereof, weclaim:

1. A process for production of a shaped article having propertiescomparable to those of conventionally forged titanium wrought articleswithout the need for subsequent sintering operations or mechanicalworking operations to substantially strengthen the article, said processcomprising hot isostatically compacting titanium powder at a temperaturein the region of but below the beta transus temperature of the titanium,employing in said compacting step a fluid which is separated from saidpowder by means of an intermediate pressure transfer medium throughwhich a substantially uniform pressure is transmitted substantiallyevenly peripherally, and maintaining in said titanium throughout theprocess a microstructure which is substantially free of alpha-outlinedprior beta grains and Widmanstatten patterned alpha particles, therebyforming a shaped article of high density and controlled interstitialcontent and having an elongation, reduction in area and strength levelscomparable to the elongation, reduction in area and strength levels ofsimilar forged titanium ingot.

ETfiJ FSEFf'ciafifi 1 wherein the interstitial content is controlled bymaintaining the oxygen content of said titanium throughout the processat not exceeding the requirement of MlLT-9047 on the filing date of thisspecification.

3. The process of claim 1 wherein the interstitial content is controlledby maintaining the nitrogen content of said titanium throughout theprocess at not exceeding the requirement of MlL-T9047 on the filing dateof this specification.

4. The iEssBFcTaim 1 wherein the inte rstitial content is controlled bymaintaining the hydrogen content of said titanium throughout the processat not exceeding the requirement of MlL-T-9047 on the tiling date ofthis specification.

5. The process of claim 1 wherein the interstitial content is controlledby maintaining the carbon content of said titanium throughout theprocess at not exceeding the requirement of MlLT9047 on the tiling dateof this specification.

6. The process o fclaim 1 whereiii the temperature at which the hotcompacting takes place is between about 200 F below the beta transustemperature and the beta transus temperature of the titanium.

7. The rdc 'r claim 1 wherein the pressure is from about 1,000 to about25,000 psi.

SITiie'prO'EES JEETm 1 vihieih the Jersey ofthe shaped article exceedsabout 97 percent of theoretical.

9. The process of claim 1 wherein the powder is compacted to at leastabout percent theoretical density by cold compacting before said hotcompacting.

a flexible mold of substantially the shape of the resulting shapedarticle with titanium powder of a density of about 15-30 percent oftheoretical and cold isostatically compacting said powder in such moldto a density of at least about percent of theoretical.

15. The process of claim 14 wherein, prior to the hot compacting, thecold-compacted powder is degassed by heating.

2. The process of claim 1 wherein the interstitial content is controlledby maintaining the oxygen content of said titanium throughout theprocess at not exceeding the requirement of MIL-T-9047 on the filingdate of this specification.
 3. The process of claim 1 wherein theinterstitial content is controlled by maintaining the nitrogen contentof said titanium throughout the process at not exceeding the requirementof MIL-T-9047 on the filing date of this specification.
 4. The processof claim 1 wherein the interstitial content is controlled by maintainingthe hydrogen content of said titanium throughout the process at notexceEding the requirement of MIL-T-9047 on the filing date of thisspecification.
 5. The process of claim 1 wherein the interstitialcontent is controlled by maintaining the carbon content of said titaniumthroughout the process at not exceeding the requirement of MIL-T-9047 onthe filing date of this specification.
 6. The process of claim 1 whereinthe temperature at which the hot compacting takes place is between about200* F below the beta transus temperature and the beta transustemperature of the titanium.
 7. The process of claim 1 wherein thecompacting pressure is from about 1,000 to about 25,000 psi.
 8. Theprocess of claim 1 wherein the density of the shaped article exceedsabout 97 percent of theoretical.
 9. The process of claim 1 wherein thepowder is compacted to at least about 60 percent theoretical density bycold compacting before said hot compacting.
 10. The process of claim 9wherein the cold compacting is followed by degassing prior to said hotcompacting.
 11. The process of claim 1 wherein the intermediate pressuretransfer medium is selected from the group consisting of molten glass,sand and steel.
 12. The process of claim 1 wherein the intermediatepressure transfer medium is solid.
 13. The process of claim 1 whereinthe intermediate pressure transfer medium is liquid.
 14. The process ofclaim 1 wherein, prior to the hot compacting, the titanium powder isprepared by filling a flexible mold of substantially the shape of theresulting shaped article with titanium powder of a density of about15-30 percent of theoretical and cold isostatically compacting saidpowder in such mold to a density of at least about 60 percent oftheoretical.
 15. The process of claim 14 wherein, prior to the hotcompacting, the cold-compacted powder is degassed by heating.